Positive electrode active material for nonaqueous electrolyte secondary battery, method for producing same, and nonaqueous electrolyte secondary battery using said positive electrode active material

ABSTRACT

Provided is a method for producing a positive electrode active material for nonaqueous electrolyte secondary batteries, including: a water-washing step of mixing, with water, Li—Ni composite oxide particles represented by the formula: LizNi1−x−yCoxMyO2 and composed of primary particles and secondary particles formed by aggregation of the primary particles to water-wash it, and performing solid-liquid separation to obtain a washed cake; a mixing step of mixing a W compound powder free from Li with the washed cake to obtain a W-containing mixture; and a heat treatment step of heating the W-containing mixture, the heat treatment step including: a first heat treatment step of heating the W-containing mixture to disperse W on the surface of the primary particles; and subsequently, a second heat treatment step of heating it at a higher temperature than in the first heat treatment step to form a lithium tungstate compound on the surface of the primary particles.

The present application is a divisional application of U.S. patentapplication Ser. No. 15/539,786, filed Jun. 26, 2017, the contents ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention. The present invention relates to a positiveelectrode active material for nonaqueous electrolyte secondary batteriesand a production method thereof, and a nonaqueous electrolyte secondarybattery using the positive electrode active material.

2. Description of the Related Art. In recent years, with the wideadoption of portable electronic devices such as mobile phones and laptopcomputers, the development of small and lightweight secondary batterieshaving high energy density is strongly desired. Further, the developmentof high power secondary batteries as batteries for electric carsincluding hybrid cars is strongly desired.

Examples of secondary batteries satisfying such demands includenonaqueous electrolyte secondary batteries typified by lithium ionsecondary batteries. Such lithium ion secondary batteries are composedof a negative electrode, a positive electrode, an electrolyte, etc., andmaterials capable of intercalation and deintercalation of lithium ionsare used for the active materials of the negative electrode and thepositive electrode.

The nonaqueous electrolyte lithium ion secondary batteries are now beingactively studied and developed. Above all, lithium ion secondarybatteries using a layered or spinel lithium-nickel composite oxide as apositive electrode material allow a high voltage of 4-V class to beobtained, and therefore are being put into practical use as batterieshaving high energy density.

Main examples of materials proposed so far include lithium cobaltcomposite oxide (LiCoO₂) that is comparatively easily synthesized,lithium nickel composite oxide (LiNiO₂) using nickel that is lessexpensive than cobalt, lithium nickel cobalt manganese composite oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂), and lithium manganese composite oxide(LiMn₂O₄) using manganese.

Among these, lithium-nickel composite oxide is gaining attention as amaterial having good cycle characteristics and low resistance andallowing high power to be obtained, where the resistance reduction thatis necessary for power enhancement has been regarded as being importantin recent years.

As a method for achieving the aforementioned resistance reduction,addition of different elements is used, and transition metals capable ofhaving high valence such as W, Mo, Nb, Ta, and Re are considered to beuseful, in particular.

For example, Japanese Patent Laid-Open No. 2009-289726 proposes alithium transition metal compound powder for lithium secondary batterypositive electrode materials containing one or more elements selectedfrom Mo, W, Nb, Ta, and Re in an amount of 0.1 to 5 mol % with respectto the total molar amount of Mn, Ni, and Co, where the total atomicratio of Mo, W, Nb, Ta, and Re with respect to the total of Li and themetal elements other than Mo, W, Nb, Ta, and Re on the surface portionsof primary particles is preferably 5 times or more the atomic ratio ofthe whole primary particles.

According to this proposal, it is considered that the cost reduction,high safety, high load characteristics, and improvement in powderhandleability of the lithium transition metal compound powder forlithium secondary battery positive electrode materials can be achievedall together.

However, the aforementioned lithium transition metal compound powder isobtained by pulverizing a raw material in a liquid medium, spray dryinga slurry in which the pulverized materials are uniformly dispersed, andfiring the obtained spray-dried material. Therefore, some of differentelements such as Mo, W, Nb, Ta, and Re are substituted with Ni disposedin layers, resulting in a reduction in battery characteristics such asbattery capacity and cycle characteristics, which has been a problem.

Further, Japanese Patent Laid-Open No. 2005-251716 proposes a positiveelectrode active material for nonaqueous electrolyte secondary batterieshaving at least a lithium transition metal composite oxide with alayered structure, wherein the lithium transition metal composite oxideis present in the form of particles composed of either or both ofprimary particles and secondary particles as aggregates of the primaryparticles, and wherein the particles have a compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine at least on the surface.

With that, it is claimed that the positive electrode active material fornonaqueous electrolyte secondary batteries having excellent batterycharacteristics even in more severe use environment is obtained, andthat the initial characteristics are improved without impairing theimprovement in thermostability, load characteristics, and outputcharacteristics particularly by having the compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine on the surface of the particles.

However, the effect by adding the at least one element selected from thegroup consisting of molybdenum, vanadium, tungsten, boron, and fluorineis to improve the initial characteristics, that is, the initialdischarge capacity and the initial efficiency, where the outputcharacteristics are not mentioned. Further, according to the disclosedproduction method, the firing is performed while the additive element ismixed with a heat-treated hydroxide together with a lithium compound,and therefore the additive element is partially substituted with nickeldisposed in layers to cause a reduction in battery characteristics,which has been a problem.

Further, Japanese Patent Laid-Open No. H11-16566 proposes a positiveelectrode active material in which the circumference of the positiveelectrode active material is coated with a metal containing at least oneselected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or anintermetallic compound obtained by combining a plurality of theseelements, and/or an oxide.

It is claimed that such coating can ensure the safety by absorbingoxygen gas, but there is no disclosure on the output characteristics.Further, the disclosed production method involves coating using aplanetary ball mill, and such a coating method causes physical damage onthe positive electrode active material, resulting in a reduction inbattery characteristics.

Further, Japanese Patent Laid-Open No. 2010-40383 proposes a positiveelectrode active material heat-treated while a tungstate compound isdeposited on composite oxide particles mainly composed of lithiumnickelate and having a carbonate ion content of 0.15 weight % or less.

According to this proposal, since the tungstate compound or adecomposition product of the tungstate compound is present on thesurface of the positive electrode active material, and the oxidationactivity on the surface of the composite oxide particles during chargeis suppressed, gas generation due to the decomposition of the nonaqueouselectrolyte or the like can be suppressed, but there is no disclosure onthe output characteristics.

Further, the disclosed production method is to deposit a solution inwhich a sulfuric acid compound, a nitric acid compound, a boric acidcompound, or a phosphate compound serving as a deposition component isdissolved in a solvent together with the tungstate compound, on thecomposite oxide particles that are preferably heated to at least theboiling point of the solution in which the deposition component isdissolved, where the solvent is removed within a short time, andtherefore the tungsten compound is not sufficiently dispersed on thesurface of the composite oxide particles and is not uniformly deposited,which has been a problem.

Further, improvements in power enhancement by lithium nickel compositeoxide have also been made.

For example, Japanese Patent Laid-Open No. 2013-125732 proposes apositive electrode active material for nonaqueous electrolyte secondarybatteries having fine particles containing lithium tungstate representedby any one of Li₂WO₄, Li₄WO₅, and Li₆W₂O₉ on the surface of alithium-nickel composite oxide composed of primary particles andsecondary particles formed by aggregation of the primary particles,where high power is supposed to be obtained together with high capacity.

Although the power is enhanced while the high capacity is maintained,further enhancement in capacity is required.

In view of such problems, it is an object of the present invention toprovide a positive electrode active material for nonaqueous electrolytesecondary batteries which allows high power together with high capacityto be obtained when used as a positive electrode material.

SUMMARY

As a result of diligent studies on the powder characteristics oflithium-nickel composite oxide used as a positive electrode activematerial for nonaqueous electrolyte secondary batteries and the effectthereof on the positive electrode resistance of the battery, for solvingthe aforementioned problems, the inventors have found that the positiveelectrode resistance of the battery can be reduced and the outputcharacteristics of the battery can be improved by forming lithiumtungstate compound on the surface of primary particles constituting thelithium-nickel composite oxide powder.

Further, as a production method thereof, they have found that thelithium tungstate compound can be formed on the surface of the primaryparticles of the lithium-nickel composite oxide by washing thelithium-nickel composite oxide with water and mixing a tungsten compoundwith the washed cake, followed by heat-treating the mixture, therebyaccomplishing the present invention.

More specifically, the first aspect of the present invention is a methodfor producing a positive electrode active material for nonaqueouselectrolyte secondary batteries, including: a water washing step ofmixing, with water, a lithium-nickel composite oxide powder representedby the general formula Li_(z)Ni_(1−x−y)CO_(x)M_(y)O₂ (where 0≤x≤0.35,0≤y≤0.35, and 0.95≤z≤1.30 are satisfied, and M is at least one elementselected from Mn, V, Mg, Mo, Nb, Ti, and Al) and having a layeredcrystal structure composed of primary particles and secondary particlesformed by aggregation of the primary particles to form a slurry, andwashing the lithium-nickel composite oxide powder with the water, andthen subjecting the slurry to solid-liquid separation to obtain a washedcake constituted by washed lithium-nickel composite oxide particles; amixing step of mixing a tungsten compound powder not containing lithiumwith the washed cake to obtain a tungsten-containing mixture; and a heattreatment step of heat-treating the obtained tungsten-containingmixture, wherein the heat treatment step includes: a first heattreatment step of heat-treating the tungsten-containing mixture to allowa lithium compound present on the surface of the primary particles ofthe washed lithium-nickel composite oxide to react with the tungstencompound so as to dissolve the tungsten compound therein, therebyforming lithium-nickel composite oxide particles with tungsten dispersedon the surface of the primary particles; and subsequent to the firstheat treatment step, a second heat treatment step of performing heattreatment at a higher temperature than in the first heat treatment stepto form lithium-nickel composite oxide particles with a lithiumtungstate compound formed on the surface of the primary particles of thelithium-nickel composite oxide.

The second aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first aspect, wherein the slurry in the waterwashing step has a concentration of 500 to 2500 g/L.

The third aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first and second aspects, wherein the slurryin the water washing step has a temperature of 20 to 30° C.

The fourth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to third aspects, wherein the washedcake obtained in the water washing step has a water content controlledto 3.0 to 15.0 mass %.

The fifth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to fourth aspects, wherein the tungstencompound not containing lithium used in the mixing step is tungstenoxide (WO₃) or tungstic acid (WO₃.H₂O).

The sixth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to fifth aspects, wherein the amount oftungsten contained in the tungsten-containing mixture is 0.05 to 2.0 at% with respect to the total number of atoms of Ni, Co, and M containedin the lithium-nickel composite oxide particles.

The seventh aspect of the present invention is the method for producinga positive electrode active material for nonaqueous electrolytesecondary batteries according to the first to sixth aspects, wherein theheat treatment step is performed in any one atmosphere of decarbonatedair, inert gas, and vacuum.

The eighth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to seventh aspects, wherein the firstheat treatment step is performed at a heat treatment temperature of 60to 80° C.

The ninth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to eighth aspects, wherein the secondheat treatment step is performed at a heat treatment temperature of 100to 200° C.

The tenth aspect of the present invention is a positive electrode activematerial for nonaqueous electrolyte secondary batteries constituted by alithium-nickel composite oxide powder having a layered crystal structurecomposed of primary particles and secondary particles formed byaggregation of the primary particles, the positive electrode activematerial being represented by the general formula:Li_(z)Ni_(1−x−y)CO_(x)M_(y)W_(a)O_(2+α) (where 0≤x≤0.35, 0≤y≤0.35,0.95≤z≤1.30, 0≤a≤0.03, and 0≤α≤0.15 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al), wherein lithiumtungstate is present on a surface of the primary particles of thelithium-nickel composite oxide, and the amount of lithium contained in alithium compound other than the lithium tungstate present on the surfaceof the primary particles of the lithium-nickel composite oxide compoundis 0.05 mass % or less with respect to the total amount of the positiveelectrode active material.

The eleventh aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the tenth aspect, wherein the amount of tungsten contained in thepositive electrode active material is 0.05 to 2.0 at % with respect tothe total number of atoms of Ni, Co, and M contained in thelithium-metal composite oxide powder.

The twelfth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the tenth aspect and the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide as fine particles having a particle sizeof 1 to 500 nm.

The thirteenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the tenth aspect and the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide as coating films having a film thicknessof 1 to 200 nm.

The fourteenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the tenth aspect and the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide in both forms of fine particles having aparticle size of 1 to 500 nm and coating films having a film thicknessof 1 to 200 nm.

The fifteenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the tenth to fourteenth aspects, wherein the content of sulfateradical is 0.05 mass % or less.

The sixteenth aspect of the present invention is a nonaqueouselectrolyte secondary battery having a positive electrode that includesthe positive electrode active material for nonaqueous electrolytesecondary batteries obtained according to the tenth to fifteenthaspects.

Advantageous Effects of Invention

According to the present invention, a positive electrode active materialfor nonaqueous electrolyte secondary batteries which can achieve highpower together with high capacity and has good cycle characteristicswhen used as a positive electrode material of a battery is obtained.Further, the production method is easy and suitable for production on anindustrial scale, and the industrial value is exceptionally large.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an equivalent circuit used formeasurement examples of impedance evaluation and analysis.

FIG. 2 is a schematic sectional view of a coin battery 1 used forbattery evaluation.

FIG. 3 is a cross-sectional SEM image (observed at 10000-foldmagnification) of a lithium-nickel composite oxide of the presentinvention.

DETAILED DESCRIPTION

Hereinafter, for the present invention, a positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention will be first described, and thereafter a production methodthereof and a nonaqueous electrolyte secondary battery using thepositive electrode active material of the present invention will bedescribed.

(1) Positive Electrode Active Material for Nonaqueous ElectrolyteSecondary Batteries

The positive electrode active material for nonaqueous electrolytesecondary batteries (which may be hereinafter referred to simply aspositive electrode active material) of the present invention isconstituted by a lithium-nickel composite oxide powder having a layeredcrystal structure composed of primary particles and secondary particlesformed by aggregation of the primary particles, wherein the compositionof the positive electrode active material is represented by the generalformula: Li_(z)Ni_(1−x−y)CO_(x)M_(y)W_(a)O_(2+α) (where 0≤x≤0.35,0≤y≤0.35, 0.95≤z≤1.30, 0<a≤0.03, and 0≤α≤0.15 are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al), lithiumtungstate is present on a surface of the primary particles of thelithium-nickel composite oxide, and the amount of lithium contained in alithium compound other than the lithium tungstate present on the surfaceof the primary particles is 0.05 mass % or less with respect to thetotal amount of the positive electrode active material.

In the present invention, high charge-discharge capacity is obtained byusing the lithium-nickel composite oxide having a compositionrepresented by the general formula: Li_(z)Ni_(1−x−y)CO_(x)M_(y)O₂ (where0≤x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al) as a base material.For obtaining higher charge-discharge capacity, x+y≤0.2 and 0.95≤z≤1.10are preferably satisfied in the aforementioned the formula.

Further, the base material is in the form of a lithium-metal compositeoxide powder constituted by primary particles and secondary particlesformed by aggregation of the primary particles (hereinafter, thesecondary particles and the primary particles existing alone may bereferred to collectively as “lithium-metal composite oxide particles”),where lithium tungstate is present on the surface of the primaryparticles of the lithium-nickel composite oxide, and the amount oflithium contained in a lithium compound other than the lithium tungstatepresent on the surface of the primary particles of the lithium-nickelcomposite oxide is 0.05 mass % or less with respect to the total amountof the positive electrode active material, thereby improving the outputcharacteristics and further improving the cycle characteristics whilemaintaining the charge-discharge capacity.

Generally, when the surface of the positive electrode active material iscompletely coated with a different compound, the movement(intercalation) of lithium ions is significantly limited, and thereforehigh capacity that is an advantage of lithium nickel composite oxide iseventually offset.

In contrast, in the present invention, lithium tungstate (which may behereinafter referred to as “LWO”) is formed on the surface of theprimary particles on the surface and inside of the lithium-nickelcomposite oxide particles, and the LWO has high lithium ion conductivityand has an effect of promoting the movement of lithium ions. Therefore,the LWO is formed on the surface of the primary particles of thelithium-nickel composite oxide, thereby forming Li conduction paths atthe interface with the electrolyte, so that the reaction resistance ofthe positive electrode active material (which may be hereinafterreferred to as “positive electrode resistance”) is reduced to improvethe output characteristics.

Thus, the reduction in positive electrode resistance reduces the voltageto be lost in the battery, and the voltage actually applied to the loadside is relatively increased, thereby allowing high power to beobtained. Further, the increase in the voltage applied to the load sideallows lithium to be sufficiently inserted into and removed from thepositive electrode, and therefore the charge-discharge capacity of thebattery (which may be hereinafter referred to as “battery capacity”) isalso improved.

Here, in the case where only the surface of the secondary particles ofthe positive electrode active material is coated with a layeredmaterial, the specific surface area decreases, regardless of the coatingthickness, and therefore even if the coating material has high lithiumion conductivity, the contact area with the electrolyte is reduced.Further, when the layered material is formed, the compound tends to beformed concentrically on a specific particle surface.

Accordingly, although the effects of improving the charge-dischargecapacity and reducing the positive electrode resistance are obtained dueto high lithium ion conductivity of the layered material serving as thecoating material, they are not sufficient, leaving room for improvement.

Further, the contact with the electrolyte occurs on the surface of theprimary particles, and therefore it is important that LWO be formed onthe surface of the primary particles.

Here, the surface of the primary particles in the present inventioninclude the surface of the primary particles exposed on the outersurface of the secondary particles, and the surface of the primaryparticles exposed into voids in the vicinity of the surface of thesecondary particles and inside thereof communicating with the outside ofthe secondary particles so as to allow the electrolyte to penetratetherethrough. Further, the surface of the primary particles includeseven the grain boundaries between the primary particles if the primaryparticles are not perfectly bonded, and the electrolyte can penetratetherethrough.

The contact with the electrolyte occurs not only on the outer surface ofthe secondary particles constituted by aggregation of the primaryparticles but also in the voids in the vicinity of the surface of thesecondary particles and inside thereof and further at the aforementionedimperfect grain boundaries, and therefore it is necessary to form LWOalso on the surface of the primary particles to promote the movement oflithium ions.

Thus, the reaction resistance of the positive electrode active materialcan be further reduced by forming LWO more on the surface of the primaryparticles which can contact with the electrolyte.

Accordingly, for obtaining higher effect of reducing the positiveelectrode resistance, LWO is preferably present on the surface of theprimary particles as fine particles having a particle size of 1 to 500nm.

The contact area with the electrolyte is rendered sufficient by havingsuch a form, so that the lithium ion conductivity can be effectivelyimproved, thereby allowing the reaction resistance of the positiveelectrode to be more effectively reduced and the charge-dischargecapacity to be improved.

When the particle size is less than 1 nm, the fine particles may fail tohave sufficient lithium ion conductivity in some cases. Further, if theparticle size is over 500 nm, the formation of the fine particles on thesurface of the primary particles may be non-uniform, resulting infailure to sufficiently obtain the effect of reducing the reactionresistance. For forming the fine particles uniformly on the surface ofthe primary particles to obtain a higher effect, the particle size ofthe fine particles is more preferably 1 to 300 nm, further preferably 5to 200 nm.

Here, LWO is not necessarily completely formed on the entire surface ofthe primary particles and may be scattered.

Even when scattered, the effect of reducing the reaction resistance isobtained as long as LWO is formed on the surface of the primaryparticles exposed on the outer surface and inside of the lithium-nickelcomposite oxide particles. Further, not all of the fine particles arenecessarily present as fine particles having a particle size of 1 to 500nm, and a high effect is obtained when 50% or more of the number of thefine particles formed on the surface of the primary particles arepreferably formed to have a particle size in the range of 1 to 500 nm.

Meanwhile, when the surface of the primary particles is coated with athin film, Li conduction paths can be formed at the interface with theelectrolyte, while the reduction in specific surface area is suppressed,and higher effects of improving the charge-discharge capacity andreducing the reaction resistance are obtained. In the case where thesurface of the primary particles are coated with LWO in the form of thinfilms as above, LWO is preferably present on the surface of the primaryparticles of the lithium-metal composite oxide as coating films with afilm thickness of 1 to 200 nm. For enhancing the effect, the filmthickness is more preferably 1 to 150 nm, further preferably 1 to 100nm.

When the film thickness is less than 1 nm, the coating films may fail tohave sufficient lithium ion conductivity in some cases. Further, whenthe film thickness is over 200 nm, the lithium ion conductivity isreduced, which may result in failure to obtain a higher effect ofreducing the reaction resistance in some cases.

However, such coating film may be partially formed on the surface of theprimary particles, and the whole coating film does not need to have afilm thickness in the range of 1 to 200 nm. When the coating film with afilm thickness of 1 to 200 nm is formed at least partially on thesurface of the primary particles, a high effect is obtained.

Further, also in the case where LWO is formed on the surface of theprimary particles is the form of fine particles as well as in the formof a coating thin film, a high effect on the battery characteristics isobtained.

Such properties of the surface of the primary particles of thelithium-nickel composite oxide can be determined, for example, byobservation using a field emission scanning electron microscope or atransmission electron microscope, and it has been confirmed that thelithium tungstate is formed on the surface of the primary particles ofthe lithium-nickel composite oxide of the positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention.

Meanwhile, in the case where LWO is non-uniformly formed between thelithium-nickel composite oxide particles, the movement of lithium ionsbetween the lithium-nickel composite oxide particles is renderednon-uniform, and therefore a load is applied onto some specificlithium-nickel composite oxide particles, which tends to cause adeterioration in cycle characteristics and an increase in reactionresistance.

Accordingly, LWO is preferably uniformly formed also between thelithium-nickel composite oxide particles.

Further, in the positive electrode active material of the presentinvention, the amount of lithium contained in a lithium compound otherthan the lithium tungstate present on the surface of the primaryparticles (which will be hereinafter referred to as excess amount oflithium) is 0.05 mass % or less, preferably 0.03 mass % or less, withrespect to the total amount of the positive electrode active material.

High charge-discharge capacity and high output characteristics areobtained and the cycle characteristics are improved by regulating theexcess amount of lithium as above.

Lithium hydroxide and lithium carbonate are present on the surface ofthe primary particles of the lithium-nickel composite oxide particles,other than the lithium tungstate, and such a lithium compound, theabundance of which can be expressed as the excess amount of lithium, haspoor conductivity of lithium and inhibits the movement of lithium ionsfrom the lithium-nickel composite oxide.

Thus, the reduction of the excess amount of lithium can enhance theeffect of promoting the movement of lithium ions by the lithiumtungstate and reduce the load on the lithium-nickel composite oxideduring charging and discharging, thereby improving the cyclecharacteristics.

Further, the control of the excess amount of lithium can make themovement of lithium ions between the lithium-nickel composite oxideparticles uniform and can suppress the application of load on specificlithium-nickel composite oxide particles, thereby improving the cyclecharacteristics.

An excessive reduction of the excess lithium means that lithium isexcessively extracted from the crystals of the lithium-nickel compositeoxide particles in forming the lithium tungstate. Accordingly, forsuppressing a reduction in battery characteristics, the excess amount oflithium is preferably 0.01 mass % or more.

Further, the content of sulfate radical (sulfate group) (which may bereferred to also as sulfate group content) in the positive electrodeactive material is preferably 0.05 mass % or less, more preferably 0.025mass % or less, further preferably 0.020 mass % or less.

If the content of sulfate radical in the positive electrode activematerial is over 0.05 mass %, an extra amount of a negative electrodematerial corresponding to the irreversible capacity of the positiveelectrode active material is inevitably used for a battery in theconstruction of the battery, resulting in a reduction in capacities bothper weight and per volume of the entire battery, and a safety problemdue to excess lithium stored in the negative electrode as anirreversible capacity, which is therefore not preferable. Further, thelower limit of the content of sulfate radical in the positive electrodeactive material is not specifically limited, but is, for example, 0.001mass % or more.

The content of sulfate radical can be determined by IPC emissionspectroscopy (ICP method) and expressing the measured amount of S(sulfur element) in terms of the amount of sulfate radical (SO₄).

The amount of tungsten contained in the lithium tungstate is 3.0 at % orless, preferably 0.05 to 2.0 at %, more preferably 0.08 to 1.0 at %,with respect to the total number of atoms of Ni, Co, and M contained inthe lithium-nickel composite oxide. The effect of improving the outputcharacteristics is obtained by adding 3.0 at % or less of tungsten.

Further, when the amount of tungsten is 0.05 to 2.0 at %, the amount ofLWO to be formed is made sufficient to reduce the positive electroderesistance and can sufficiently ensure the surface area of the primaryparticles capable of contacting with the electrolyte, and both highcharge-discharge capacity and high output characteristics can be furtherachieved.

When the amount of tungsten is less than 0.05 at %, the effect ofimproving the output characteristics may fail to be sufficientlyobtained, and when the amount of tungsten is over 2.0 at %, the amountof the compound formed excessively increases to inhibit the conductionof lithium ions between the lithium-nickel composite oxide and theelectrolyte, which may result in a reduction in charge-dischargecapacity.

Further, as the amount of lithium in the entire positive electrodeactive material, the atomic ratio “Li/Me” of the number of atoms of Liwith respect to the sum of the number of atoms of Ni, Co, and M in thepositive electrode active material (Me) is 0.95 to 1.30, preferably 0.97to 1.25, more preferably 0.97 to 1.20. Thus, the ratio Li/Me in thelithium-metal composite oxide particles as a core material is set topreferably 0.95 to 1.25, more preferably 0.95 to 1.20, thereby allowinghigh battery capacity to be obtained and allowing the amount of lithiumthat is sufficient to form LWO to be ensured. Here, the core materialrefers to the lithium-metal composite oxide particles excluding the LWcompound, and the positive electrode active material is obtained byforming the LW compound on the surface of the primary particles of thelithium-metal composite oxide particles.

When the ratio Li/Me is less than 0.95, the reaction resistance of thepositive electrode in the nonaqueous electrolyte secondary battery usingthe obtained positive electrode active material increases, and thus theoutput of the battery decreases. Further, when the ratio Li/Me is over1.30, the initial discharge capacity of the positive electrode activematerial decreases, and the reaction resistance of the positiveelectrode increases as well. The content of lithium in the LWO issupplied from the lithium-nickel composite oxide particles serving asthe base material, and therefore the amount of lithium in the entirepositive electrode active material does not change before and after theformation of the LWO.

That is, the ratio Li/Me in the lithium-nickel composite oxide particlesserving as the core material decreases after the formation of the LWO ascompared with before its formation. Therefore, better charge-dischargecapacity and lower reaction resistance can be obtained by setting theratio Li/Me in the entire positive electrode active material at 0.97 ormore.

Accordingly, for obtaining higher battery capacity, the amount oflithium in the entire positive electrode active material is morepreferably 0.97 to 1.15. Further, the ratio Li/Me of the lithium-nickelcomposite oxide particles serving as the core material is morepreferably 0.95 to 1.15, further preferably 0.95 to 1.10.

The positive electrode active material of the present invention hasoutput characteristics and cycle characteristics improved by providinglithium tungstate on the surface of the primary particles of thelithium-nickel composite oxide particles, and the powder characteristicsas the positive electrode active material such as particle size and tapdensity need only to fall within the range of commonly used positiveelectrode active materials.

Further, the effect by providing the lithium tungstate on the surface ofthe primary particles of the lithium-nickel composite oxide particles isapplicable, for example, not only to powders of lithium-cobalt compositeoxide, lithium-manganese composite oxide,lithium-nickel-cobalt-manganese composite oxide, and the like, and thepositive electrode active material described in the present invention,but also to commonly used positive electrode active materials forlithium secondary batteries.

(2) Method for Producing Positive Electrode Active Material

Hereinafter, a method for producing the positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention will be described in detail for each step.

[Water Washing Step]

The water washing step is a step of obtaining a washed cake composed ofwashed lithium-nickel composite oxide particles by mixing, with water, alithium-nickel composite oxide powder as a base material having acomposition represented by the formula Li_(z)Ni_(1−x−y)CO_(x)M_(y)O₂(where 0≤x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30 are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al) andhaving a layered crystal structure composed of primary particles andsecondary particles formed by aggregation of the primary particles toform a slurry, followed by washing with water, filtration, andsolid-liquid separation.

In lithium-nickel composite oxide powder, particularly, thelithium-nickel composite oxide powder obtained by sintering nickelcomposite hydroxide or nickel composite oxide with a lithium compound,an unreacted lithium compound is present on the surface of the secondaryparticles or the primary particles.

Accordingly, the excess of the unreacted lithium compound such aslithium hydroxide and lithium carbonate, sulfate radical, and otherimpurity elements, which deteriorate the battery characteristics, can beremoved from the lithium-nickel composite oxide particles by washingwith water.

Further, in the water washing step, water necessary for promoting thereaction between the lithium compound present on the surface of theprimary particles of the lithium-nickel composite oxide and the tungstencompound can be given to the lithium-nickel composite oxide powder.

When washing the lithium-nickel composite oxide powder with water asabove, the slurry concentration is preferably 500 to 2500 g/L, morepreferably 750 to 2000 g/L. Here, the slurry concentration “g/L” meansthe amount “gram” of the lithium-nickel composite oxide particles to bemixed with 1 L of water.

If the slurry concentration is less than 750 g/L, the lithium compoundthat is present on the surface of the lithium-nickel composite oxideparticles and is necessary for the reaction with the tungsten compoundmay also be washed away, and in the subsequent step, the reactionbetween the lithium compound and the tungsten compound may notsufficiently proceed.

Meanwhile, if the slurry concentration is over 1500 g/L, the unreactedlithium compound and the impurity elements may remain more thannecessary, deteriorating the battery characteristics.

The water washing temperature is preferably 10 to 40° C., morepreferably 20 to 30° C. If the water washing temperature is lower than10° C., the lithium compound may remain more than necessary,deteriorating the battery characteristics. If the water washingtemperature is higher than 40° C., the lithium compound may be washedaway excessively.

The water washing time is not specifically limited, but is preferablyabout 5 to 60 minutes. If the water washing time is too short, thelithium compound and the impurities on the surface of the lithium-nickelcomposite oxide particles may not be sufficiently removed but remainthere. Meanwhile, even if the water washing time is longer than notedabove, the washing effect is not improved, and the productivity ratherdecreases.

The water used for forming the slurry is not specifically limited, butwater with an electric conductivity as measured of less than 10 μS/cm ispreferable, and water with an electric conductivity as measured of 1μS/cm or less is more preferable, for preventing a reduction in batterycharacteristics due to deposition of impurities onto the positiveelectrode active material.

The method for solid-liquid separation after the washing with water isnot specifically limited, and commonly used devices and methods areused. For example, a suction filter, a centrifuge, a filter press, orthe like is preferably used.

Here, the cake composed of the washed lithium-nickel composite oxideparticles obtained by the solid-liquid separation after the washing withwater, that is, the washed cake has a water content of preferably 2.0mass % or more, more preferably 3.0 to 15.0 mass %, more preferably 6.5to 11.5 mass %.

At the water content of 2.0 mass % or more, the amount of waternecessary for promoting the reaction between the lithium compoundpresent on the surface of the washed lithium-nickel composite oxideparticles and the tungsten compound can be more sufficient. Such asufficient amount of water allows the tungsten compound to be dissolvedand tungsten contained in the tungsten compound to penetrate into voidsbetween the primary particles communicating with the outside of thesecondary particles and incomplete grain boundaries together with water,so that a sufficient amount of tungsten can be dispersed on the surfaceof the primary particles.

Further, the water content is more preferably 3.0 to 15.0 mass %,further preferably 6.5 to 11.5 mass %, which can facilitate the mixingof the tungsten compound by suppressing an increase in viscosity of thewashed cake when being slurried and can further improve the productivityby reducing the drying time. Further, when the positive electrode activematerial to be obtained is used as a positive electrode of a battery,deterioration in battery characteristics due to an increase in elutionof lithium from the lithium-nickel composite oxide particles can befurther suppressed.

[Mixing Step]

The mixing step is a step of obtaining a tungsten-containing mixturewith the lithium-nickel composite oxide particles constituting thewashed cake (which will be hereinafter referred to simply as “mixture”)by mixing a tungsten compound powder free from lithium with the washedcake obtained in the water washing step.

The tungsten compound to be used is preferably water-soluble so as to bedissolved in water contained in the mixture, in order to penetrate tothe surface of the primary particles inside the secondary particles.Further, the water in the mixture becomes alkaline due to the elution oflithium, and therefore the tungsten compound may be a compound solublein alkaline water. Further, the mixture is heated in the subsequent heattreatment step, and therefore, even if the tungsten compound isdifficult to be dissolved in water at room temperature, it needs only tobe dissolved in water by heating in the heat treatment or by forminglithium tungstate through the reaction with the lithium compound on thesurface of the lithium-nickel composite oxide particles.

Further, the dissolved tungsten compound needs only to be in an amountthat allows the penetration to the surface of the primary particlesinside the secondary particles, and therefore the tungsten compound maybe partially solid after the mixing or after the heating.

In this way, the tungsten compound needs only to be free from lithiumand to be soluble in water when heated in the heat treatment step, andtungsten oxide, tungstic acid, ammonium tungstate, sodium tungstate, orthe like, is preferable, and tungsten oxide (WO₃) or tungstic acid(WO₃.H₂O), in which the possibility of impurity contamination is low, ismore preferable.

Further, the amount of tungsten contained in the mixture is preferably3.0 at % or less, more preferably 0.05 to 3.0 at %, further preferably0.05 to 2.0 at %, particularly preferably 0.08 to 1.0 at %, with respectto the total number of atoms of Ni, Co, and M contained in thelithium-nickel composite oxide particles.

Thereby, the amount of tungsten contained in the lithium tungstate inthe positive electrode active material is adjusted to the preferablerange, and both high charge-discharge capacity and high outputcharacteristics of the positive electrode active material can be furtherachieved.

Further, the washed cake is preferably mixed with the tungsten compoundat a temperature of 50° C. or less.

If the temperature is over 50° C., the resulting mixture may not have awater content necessary for promoting the reaction between the lithiumcompound and the tungsten compound because they may be dried during themixing.

For mixing the washed cake of the lithium-nickel composite oxide and thetungsten compound powder, a common mixer can be used. For example, themixing may be performed sufficiently to an extent such that the shape ofthe lithium-nickel composite oxide particles is not broken, using ashaker mixer, a Loedige mixer, a Julia mixer, a V blender, or the like.

[Heat Treatment Step]

The heat treatment step is a step of heat-treating thetungsten-containing mixture and includes: a first heat treatment step ofdispersing tungsten on the surface of the primary particles by allowingthe lithium compound present on the surface of the primary particles ofthe lithium-nickel composite oxide to react with the tungsten compoundso as to dissolve the tungsten compound therein; and a second heattreatment step of forming a lithium tungstate compound on the surface ofthe primary particles of the lithium-nickel composite oxide byperforming heat treatment at a higher temperature than the heattreatment temperature in the first heat treatment step.

Here, use of the tungsten compound free from lithium and the first heattreatment step of dispersing tungsten on the surface of the primaryparticles by allowing the lithium compound to react with the tungstencompound so as to dissolve the tungsten compound therein are important.

In the first heat treatment step, heating the mixture containing thetungsten compound free from lithium allows not only lithium eluted inthe mixture but also the lithium compound remaining on the surface ofthe primary particles of the lithium-nickel composite oxide particles toreact with the tungsten compound to form the lithium tungstate. Theformation of lithium tungstate can improve the battery characteristicsby considerably reducing the excess lithium in the positive electrodeactive material to be obtained.

Further, there is also an effect of extracting excess lithium present inthe lithium-nickel composite oxide particles, and the extracted lithiumreacts with the tungsten compound to contribute to improvement incrystallinity of the lithium-nickel composite oxide particles when theyserve as a positive electrode active material, so that the batterycharacteristics can be further enhanced.

The lithium tungstate formed by such a reaction is dissolved in water inthe mixture and penetrates into the voids between the primary particlesinside the secondary particles and the incomplete grain boundaries, sothat tungsten can be dispersed on the surface of the primary particles.

For dispersing tungsten by allowing the lithium compound to react withthe tungsten compound as described above, it is preferable that waterremain until the reaction sufficiently proceeds and tungsten penetratestherein.

Accordingly, the first heat treatment step is performed at a heattreatment temperature of preferably 60 to 80° C.

If the temperature is less than 60° C., the lithium compound present onthe surface of the primary particles of the lithium-nickel compositeoxide may not sufficiently react with the tungsten compound tosynthesize a necessary amount of lithium tungstate. Meanwhile, if thetemperature is higher than 80° C., water may evaporate too rapidly tosufficiently progress the reaction of the lithium compound present onthe surface of the primary particles with the tungsten compound and thepenetration of tungsten.

The heating time in the first heat treatment step is not specificallylimited, but is preferably 0.5 to 2 hours for allowing the lithiumcompound to react with the tungsten compound and tungsten tosufficiently penetrate therein.

The second heat treatment step is a step of forming a lithium tungstatecompound on the surface of the primary particles of the lithium-nickelcomposite oxide particles by performing heat treatment at a highertemperature than the heat treatment temperature in the first heattreatment step to sufficiently evaporate water in the mixture, and theheat treatment temperature is preferably 100 to 200° C.

If the temperature is less than 100° C., the evaporation of water may beinsufficient to sufficiently form the lithium tungstate compound.Meanwhile, if the temperature is over 200° C., necking between thelithium-nickel composite oxide particles may be formed via the lithiumtungstate and/or the specific surface area of the lithium-nickelcomposite oxide particles may be reduced significantly, leading todegradation of the battery characteristics.

The heat treatment time in the second heat treatment step is notspecifically limited, but is preferably 1 to 15 hours, more preferably 5to 12 hours, for sufficiently evaporating water to form the lithiumtungstate compound.

The heat treatment step is preferably performed in a decarboxylation airatmosphere, an inert gas atmosphere, or a vacuum atmosphere, foravoiding the reaction of water or carbonic acid in the atmosphere withlithium on the surface of the lithium-nickel composite oxide particles.

(3) Nonaqueous Electrolyte Secondary Battery

The nonaqueous electrolyte secondary battery of the present invention isconstituted by a positive electrode, a negative electrode, a nonaqueouselectrolyte, etc., and constituted by the same components as those ofcommon nonaqueous electrolyte secondary batteries. The embodimentdescribed below is just an example, and the nonaqueous electrolytesecondary battery of the present invention can be implemented byemploying embodiments in which various changes and improvements aremade, using the embodiment shown in this description as a base, based onthe knowledge of those skilled in the art. Further, the applications ofthe nonaqueous electrolyte secondary battery of the present inventionare not specifically limited.

(a) Positive Electrode

Using the positive electrode active material for nonaqueous electrolytesecondary batteries described above, the positive electrode of thenonaqueous electrolyte secondary battery is produced, for example, asfollows.

First, a positive electrode active material in powder form, a conductivematerial, and a binder are mixed, and activated carbon and a solvent forits intended purpose such as a viscosity adjuster are further added, asneeded, and the mixture is kneaded to produce a positive electrodecomposite material paste.

The mixing ratio of each component in the positive electrode compositematerial paste is also an important element to determine the performanceof the nonaqueous electrolyte secondary battery. When the total mass ofthe solid contents of the positive electrode composite materialexcluding the solvent is taken as 100 parts by mass, it is preferablethat the content of the positive electrode active material be 60 to 95parts by mass, the content of the conductive material be 1 to 20 partsby mass, and the content of the binder be 1 to 20 parts by mass, as in apositive electrode of a common nonaqueous electrolyte secondary battery.

The obtained positive electrode composite material paste, for example,is applied to the surface of a current collector made of aluminum foil,followed by drying, to disperse the solvent. In order to enhance theelectrode density, it may be pressed by roll pressing or the like, asneeded. Thus, a positive electrode in sheet form can be produced.

The positive electrode in sheet form can be used for producing abattery, for example, by being cut into a suitable size corresponding tothe intended battery. However, the method for producing the positiveelectrode is not limited to the aforementioned example, and anothermethod may be employed.

For producing the positive electrode, graphite (such as naturalgraphite, artificial graphite, and expanded graphite) and carbon blackmaterials such as acetylene black and Ketjen black (R), for example, canbe used as the conductive material.

The binder serves to hold the active material particles, for whichpolyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),fluororubber, ethylene propylene diene rubber, styrene butadiene,cellulose resins, and polyacrylic acid, for example, can be used.

As needed, the positive electrode active material, the conductivematerial, and the activated carbon are dispersed, and a solvent todissolve the binder is added to the positive electrode compositematerial.

Specifically, an organic solvent such as N-methyl-2-pyrrolidone can beused as the solvent. Further, activated carbon can be added to thepositive electrode composite material for increasing the capacity of theelectric double layer.

(b) Negative Electrode

As the negative electrode, a material formed by applying a negativeelectrode composite material formed into a paste by mixing the binderwith metal lithium, lithium alloy, or the like, or a negative electrodeactive material capable of absorbing and desorbing lithium ions andadding a suitable solvent onto the surface of the current collector madeof a metal foil such as copper, followed by drying and compressing forincreasing the electrode density, as needed, is used.

As the negative electrode active material, a powder material of naturalgraphite, artificial graphite, a fired material of an organic compoundsuch as a phenolic resin, and a carbon material such as cokes, forexample, can be used. In this case, a fluorine-containing resin such asPVDF can be used as the negative electrode binder, as in the positiveelectrode, and an organic solvent such as N-methyl-2-pyrrolidone can beused as the solvent to disperse the active material and the bindertherein.

(c) Separator

A separator is interposed between the positive electrode and thenegative electrode.

The separator separates the positive electrode and the negativeelectrode from each other and holds the electrolyte. A thin film ofpolyethylene, polypropylene, or the like having a large number of fineholes can be used as the separator.

(d) Non-Aqueous Electrolyte

The nonaqueous electrolyte is formed by dissolving a lithium salt as asupporting salt in an organic solvent.

As the organic solvent used, one selected from cyclic carbonates such asethylene carbonate, propylene carbonate, butylene carbonate, andtrifluoropropylene carbonate, chain carbonates such as diethylcarbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropylcarbonate, ether compounds such as tetrahydrofuran,2-methyltetrahydrofuran, and dimethoxyethane, sulfur compounds such asethyl methyl sulfone and butanesulton, and phosphorus compounds such astriethyl phosphate and trioctyl phosphate can be used alone, or two ormore of these can be mixed for use.

As the supporting salt, LiPF₆, LiBF₄, LiClO₄, LiAsF₆, and LiN(CF₃SO₂)₂,and composite salts of these can be used.

Further, the non-aqueous electrolyte may contain a radical scavenger, asurfactant, a flame retardant, and the like.

(e) Shape and Configuration of Battery

The nonaqueous electrolyte secondary battery of the present inventionconstituted by the positive electrode, the negative electrode, theseparator, and the non-aqueous electrolyte described above can havevarious shapes such as a cylindrical type and a stacked type.

Even if any shape is employed, an electrode body is obtained by stackingthe positive electrode and the negative electrode via the separator, theobtained electrode body is impregnated with the non-aqueous electrolyte,the connection between the positive electrode current collector and thepositive electrode terminal connected to the outside and the connectionbetween the negative electrode current collector and the negativeelectrode terminal connected to the outside are established using leadsfor the current collectors, and the components are sealed in a batterycase, to complete the nonaqueous electrolyte secondary battery.

(f) Characteristics

The nonaqueous electrolyte secondary battery using the positiveelectrode active material of the present invention has high capacity andhigh power.

In particular, the nonaqueous electrolyte secondary battery obtained bya further preferable embodiment using the positive electrode activematerial according to the present invention, for example, when used as apositive electrode of a 2032-type coin battery, has a high initialdischarge capacity of 165 mAh/g or more and a low positive electroderesistance and further has high capacity and high power. Further, italso has high thermostability and excellent safety.

The method for measuring the positive electrode resistance in thepresent invention is exemplified, as follows.

When the frequency dependence of a battery reaction is measured by acommon AC impedance method as an electrochemical evaluation technique, aNyquist diagram based on the solution resistance, the negative electroderesistance and the negative electrode capacity, and the positiveelectrode resistance and the positive electrode capacity is obtained asshown in FIG. 1.

The battery reaction in an electrode is made by the resistancecomponents following charge transfers and the capacity components by anelectric double layer. When these components are shown as an electricalcircuit, a parallel circuit of the resistance and the capacity isobtained, and they are shown as an equivalent circuit in which thesolution resistance and the parallel circuit of the negative electrodeand the positive electrode are connected in series as the entirebattery.

The Nyquist diagram determined is subjected to fitting calculation usingthe equivalent circuit, and the resistance components and the capacitycomponents each can be estimated.

The positive electrode resistance is equal to the diameter of asemicircle on the low frequency side of the Nyquist diagram to beobtained.

From above, the positive electrode resistance can be estimated byperforming the AC impedance measurement on the produced positiveelectrode and subjecting the obtained Nyquist diagram to fittingcalculation using the equivalent circuit.

EXAMPLES

For a secondary battery having a positive electrode using the positiveelectrode active material obtained by the present invention, theperformance (such as initial discharge capacity, positive electroderesistance and cycle characteristics) was measured.

Hereinafter, the present invention will be specifically described by wayof examples, but the present invention is not limited to these examplesat all.

(Production and Evaluation of Battery)

For evaluating the positive electrode active material, a 2032-type coinbattery 1 (which will be hereinafter referred to as coin type battery)shown in FIG. 2 was used.

As shown in FIG. 2, the coin type battery 1 is constituted by a case 2and electrodes 3 housed in the case 2.

The case 2 has a hollow positive electrode can 2 a with one end open anda negative electrode can 2 b arranged in the opening of the positiveelectrode can 2 a, and is configured so that, when the negativeelectrode can 2 b is arranged in the opening of the positive electrodecan 2 a, a space to house the electrodes 3 is formed between thenegative electrode can 2 b and the positive electrode can 2 a.

The electrodes 3 are constituted by a positive electrode 3 a, aseparator 3 c, and a negative electrode 3 b, which are stacked to bealigned in this order and are housed in the case 2 so that the positiveelectrode 3 a is in contact with the inner surface of the positiveelectrode can 2 a, and the negative electrode 3 b is in contact with theinner surface of the negative electrode can 2 b.

The case 2 includes a gasket 2 c, and the relative movement between thepositive electrode can 2 a and the negative electrode can 2 b is fixedby the gasket 2 c so that the non-contact state is maintained. Further,the gasket 2 c also has a function of sealing the gap between thepositive electrode can 2 a and the negative electrode can 2 b so as toblock between the inside and the outside of the case 2 air-tightly andliquid-tightly.

The coin type battery 1 shown in FIG. 2 was fabricated as follows.

First, 52.5 mg of the positive electrode active material for nonaqueouselectrolyte secondary batteries, 15 mg of acetylene black, and 7.5 mg ofpolytetrafluoroethylene resin (PTFE) were mixed, followed by pressmolding at a pressure of 100 MPa to a diameter of 11 mm and a thicknessof 100 μm, to produce the positive electrode 3 a. The thus producedpositive electrode 3 a was dried in a vacuum dryer at 120° C. for 12hours.

Using the positive electrode 3 a, the negative electrode 3 b, theseparator 3 c, and the electrolyte, the coin type battery 1 shown inFIG. 2 was produced in a glove box under Ar atmosphere with the dewpoint controlled to −80° C.

As the negative electrode 3 b, a negative electrode sheet in whichgraphite powder with an average particle size of about 20 μm andpolyvinylidene fluoride were applied to a copper foil and which waspunched into a disk shape with a diameter of 14 mm was used.

As the separator 3 c, a polyethylene porous film with a film thicknessof 25 μm was used. As the electrolyte, an equal mixture (manufactured byTOMIYAMA PURE CHEMICAL INDUSTRIES, LTD.) of ethylene carbonate (EC) anddiethyl carbonate (DEC) with 1 M LiClO₄ serving as a supportingelectrolyte was used.

The initial discharge capacity and the positive electrode resistanceshowing the performance of the thus produced coin type battery 1 wereevaluated as follows.

The capacity when the coin type battery 1 allowed to stand for about 24hours from the fabrication was charged, with the current density withrespect to the positive electrode set to 0.1 mA/cm², to a cut-offvoltage of 4.3 V after the OCV (Open Circuit Voltage) became stable,followed by a pause for one hour, and was discharged to a cut-offvoltage of 3.0 V was taken as the initial discharge capacity.

The Nyquist plot shown in FIG. 1 is obtained by charging the coin typebattery 1 at a charge potential of 4.1 V and measuring the positiveelectrode resistance using a frequency response analyzer and apotentio-galvanostat (1255B, manufactured by Solartron) by the ACimpedance method.

Since the Nyquist plot is shown as the sum of characteristic curvesshowing the solution resistance, the negative electrode resistance andthe capacity thereof, and the positive electrode resistance and thecapacity thereof, fitting calculation was performed based on the Nyquistplot using the equivalent circuit to calculate the value of the positiveelectrode resistance.

The cycle characteristics were evaluated based on the capacity retentionrate and the rate of increase in positive electrode resistance after acycle test. The cycle test was performed by measuring the initialdischarge capacity, followed by a 10-minute pause, and thereafterrepeating the charge and discharge cycle, in the same manner as in themeasurement of the initial discharge capacity, 500 times (charge anddischarge) including the measurement of the initial discharge capacity.The discharge capacity at the 500th cycle was measured, and thepercentage of the discharge capacity at the 500th cycle with respect tothe discharge capacity (initial discharge capacity) at the 1st cycle wasdetermined as capacity retention rate (%). Further, the positiveelectrode resistance after the 500 cycles was measured and was evaluatedbased on the rate of increase (fold) from the positive electroderesistance before the cycle test.

In the present examples, the positive electrode active material, and thesecondary battery, the respective samples of special reagentsmanufactured by Wako Pure Chemical Industries, Ltd. were used forproducing the composite hydroxide.

Example 1

A powder of lithium-nickel composite oxide particles represented byLi_(1.025)Ni_(0.91)Co_(0.06)Al_(0.03)O₂ and obtained by a knowntechnique of mixing an oxide containing Ni as a main component andlithium hydroxide followed by firing was used as a base material.

100 mL of pure water at 25° C. was added to 150 g of the base materialto form a slurry, followed by washing with water for 15 minutes. Afterthe washing with water, solid-liquid separation was performed byfiltration using a Buchner funnel. The washed cake had a water contentof 8.5 mass %.

Next, 1.08 g of tungsten oxide (WO₃) was added to the washed cake sothat the amount of W was 0.30 at % with respect to the total number ofatoms of Ni, Co, and Al contained in the lithium-nickel composite oxide,and the mixture was sufficiently mixed using a shaker mixer (TURBULATypeT2C, manufactured by Willy A. Bachofen AG) to obtain a mixed powder.

The obtained mixed powder was put into an aluminum bag, which was purgedwith a nitrogen gas, followed by lamination, and was put into a dryerheated to 80° C. for about 1 hour. After the heating, it was taken outof the aluminum bag and was replaced into a SUS container, followed bystatic drying using a vacuum dryer heated to 190° C. for 10 hours andthereafter cooling in a furnace.

Finally, a sieve with a mesh opening of 38 μm was applied fordeagglomeration, to obtain a positive electrode active material having alithium tungstate compound on the surfaces of the primary particles.

The obtained positive electrode active material was analyzed by the ICPmethod, and it was confirmed that the tungsten content was 0.30 at %with respect to the total number of atoms of Ni, Co, and Al, and theratio Li/Me was 0.99. Further, the content of sulfate radical asdetermined by conversion from the sulfur content measured by the ICPmethod was 0.01 mass %.

[Analysis of Excess Lithium]

Excess lithium in the obtained positive electrode active material wasevaluated by titrating Li eluted from the positive electrode activematerial. As a result of evaluating the excess amount of lithium byanalyzing the compound state of lithium eluted from the neutralizationpoint appearing by adding pure water to the obtained positive electrodeactive material, followed by stirring for a certain time, and thereafteradding hydrochloric acid while measuring the pH of the filtrate afterfiltration, the excess amount of lithium was 0.02 mass % with respect tothe total amount of the positive electrode active material.

[Morphological Analysis of Lithium Tungstate]

The obtained positive electrode active material was embedded into aresin, and cross-section polishing was performed thereon to produce asample for observation. The cross section of the sample was observed bySEM at 5000-fold magnification, and it was confirmed that the sample wasconstituted by primary particles and secondary particles formed byaggregation of the primary particles, fine particles of lithiumtungstate were formed on the surface of the primary particles thereof.The fine particles had a particle size of 20 to 150 nm.

Further, it was confirmed that 85% of the number of the observedsecondary particles had lithium tungstate formed on the surface of theprimary particles, and the lithium tungstate was uniformly formedbetween the secondary particles.

Further, the vicinity of the surface of the primary particles of theobtained positive electrode active material was observed by atransmission electron microscope (TEM), and it was confirmed thatcoating films with a film thickness of 2 to 65 nm were formed on thesurface of the primary particles, and the coating films were the lithiumtungstate.

[Evaluation of Battery]

The battery characteristics of the coin type battery 1 shown in FIG. 2having a positive electrode produced using the obtained positiveelectrode active material were evaluated. The positive electroderesistance before the cycle test was shown as a relative value, takingthe evaluation value of Example 1 as “1.00”. The initial dischargecapacity was 216 mAh/g.

Hereinafter, for Examples and Comparative Examples, only materials andconditions changed from those in Example 1 above are shown. Further,Table 1 shows the conditions in Example 1 from the water washing step tothe heat treatment step, and Table 2 shows the evaluation results.

Example 2

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 0.52 g of tungsten oxide(WO₃) was added to the washed cake so that the amount of W was 0.15 at %with respect to the total number of atoms of Ni, Co, and Al contained inthe lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Example 3

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 0.36 g of tungsten oxide(WO₃) was added to the washed cake so that the amount of W was 0.10 at %with respect to the total number of atoms of Ni, Co, and Al contained inthe lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Example 4

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 150 mL of pure water at 25°C. was added to 150 g of the base material to form a slurry, and 0.54 gof tungsten oxide (WO₃) was added to the washed cake so that the amountof W was 0.15 at % with respect to the total number of atoms of Ni, Co,and Al contained in the lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Example 5

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 200 mL of pure water at 25°C. was added to 150 g of the base material to form a slurry, and 0.54 gof tungsten oxide (WO₃) was added to the washed cake so that the amountof W was 0.15 at % with respect to the total number of atoms of Ni, Co,and Al contained in the lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Example 6

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 100 mL of pure water at 40°C. was added to 150 g of the base material to form a slurry, and 0.54 gof tungsten oxide (WO₃) was added to the washed cake so that the amountof W was 0.15 at % with respect to the total number of atoms of Ni, Co,and Al contained in the lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Example 7

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that the conditions forfiltration using the Buchner funnel after the washing with water wereadjusted so that the washed cake after the solid-liquid separation had awater content of 2.5 mass %, and 0.54 g of tungsten oxide (WO₃) wasadded to the washed cake so that the amount of W was 0.15 at % withrespect to the total number of atoms of Ni, Co, and Al contained in thelithium-nickel composite oxide.

Table 1 and Table 2 show the results.

Comparative Example 1

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 200 mL of pure water at 25°C. was added to 150 g of the base material to form a slurry, and thetungsten compound was not added to the washed cake after thesolid-liquid separation.

Table 1 and Table 2 show the results.

Comparative Example 2

A positive electrode active material was obtained and was evaluated inthe same manner as in Example 1 except that 200 mL of pure water at 25°C. was added to 150 g of the base material to form a slurry, and lithiumtungstate (LWO:Li₂WO₄) was added to the washed cake so that the amountof W was 0.15 at % with respect to the total number of atoms of Ni, Co,and Al contained in the lithium-nickel composite oxide.

Table 1 and Table 2 show the results.

TABLE 1 First heat Second heat Slurry Water Water treatment treatmentconcentration temperature content temperature temperature [g/L] [° C.][mass %] W source*¹ [° C.] [° C.] Example 1 1500 25 8.5 WO₃ 80 190Example 2 1500 25 8.5 WO₃ 80 190 Example 3 1500 25 8.5 WO₃ 80 190Example 4 1000 25 8.2 WO₃ 80 190 Example 5 750 25 8.2 WO₃ 80 190 Example6 1500 40 8.3 WO₃ 80 190 Example 7 1500 25 2.5 WO₃ 80 190 Comparative750 25 9.5 — 80 190 Example 1 Comparative 750 25 8.5 LWO 80 190 Example2 *¹Although LWO had no significant difference in initial batterycharacteristics from WO₃, the battery characteristics decreased aftercycles (500 cycles).

TABLE 2 After 500 cycles Positive Rate of Content of LWO on surface ofprimary particles Initial electrode Capacity increase in sulfateParticle Film discharge resistance retention positive Amount of Wradical Excess Li size thickness capacity before rate electrode [mass %][mass %] [mass %] Form [nm] [nm] [mAh/g] cycles *² [%] resistance *³Example 1 0.30 0.01 0.02 Thin film + 20 to 150 2 to 65 216 1.00 85 12Fine particles Example 2 0.15 0.01 0.02 Thin film + 10 to 120 1 to 55218 0.91 86 12 Fine particles Example 3 0.10 0.01 0.03 Thin film — 1 to55 217 0.91 86 12 Example 4 0.15 0.01 0.02 Thin film + 20 to 160 2 to 60219 0.95 87 11 Fine particles Example 5 0.15 0.02 0.02 Thin film — 2 to70 215 1.00 85 12 Example 6 0.15 0.01 0.005 Thin film + 10 to 145 1 to55 215 1.00 83 12 Fine particles Example 7 0.15 0.01 0.04 Thin film + 10to 140 1 to 55 215 1.02 82 13 Fine particles Comparative 0.00 0.02 0.03— — — 204 4.32 73 45 Example 1 Comparative 0.15 0.02 0.06 Thin film + 20to 215  2 to 105 215 1.09 81 15 Example 2 Fine particles *² Reactionresistance before cycles was calculated taking Example 1 as 1.00. *³Rate of increase in reaction resistance = Reaction resistance aftercycles/Reaction resistance before cycles

[Evaluation]

As is obvious from Table 1 and Table 2, the positive electrode activematerials of Examples 1 to 7 were produced according to the presentinvention and therefore had high initial discharge capacity and lowpositive electrode resistance, as compared with Comparative Examples,and they formed batteries having good cycle characteristics andexcellent characteristics.

Further, FIG. 3 shows an example of the results of the cross sectionalSEM observation of the positive electrode active materials obtained inExamples of the present invention, where it was confirmed that such apositive electrode active materials thus obtained was composed ofprimary particles and secondary particles formed by aggregation of theprimary particles, and lithium tungstate was formed on the surface ofthe primary particles. FIG. 3 shows the positions where the lithiumtungstate compound was observed by circles.

In Example 6, since the water washing temperature was as high as 40° C.,the washed amount of the lithium compound present on the surface of theprimary particles of the lithium-nickel composite oxide necessary forthe reaction with the tungsten compound increased, and lithium extractedfrom the crystals of the lithium-nickel composite oxide in forming thelithium tungstate compound increased to reduce the crystallinity,resulting in a slight decrease in capacity retention rate in the cycletest.

In Example 7, since the washed cake had low water content, the reactionof the lithium compound present on the surface of the primary particlesof the lithium-nickel composite oxide with the tungsten compound wasinsufficient, the formation of the lithium tungstate compound wasreduced, the dispersibility decreased, and further excess lithiumslightly increased, therefore resulting in a slight reduction in outputcharacteristics and battery characteristics in the cycle test.

In contrast, in Comparative Example 1, since the lithium tungstateaccording to the present invention was not formed on the surface of theprimary particles, the positive electrode resistance was considerablyhigh, and thus it is difficult to satisfy the requirements of powerenhancement.

Further, in Comparative Example 2, since lithium tungstate was added tothe washed cake, excess lithium in the positive electrode activematerial increased, resulting in a reduction in output characteristicsand battery characteristics in the cycle test.

The nonaqueous electrolyte secondary battery of the present invention issuitable for power sources of small portable electronic devices (such aslaptop personal computers and mobile phone terminals) that constantlyrequire high capacity and is suitable for batteries for electric carsthat require high power.

Further, the nonaqueous electrolyte secondary battery of the presentinvention has excellent safety and allows size reduction and powerenhancement, and therefore it is suitable as a power source for electriccars where there is a restriction on the mounting space. The presentinvention can be used not only as a power source for electric cars whichare purely driven by electric energy but also as a power source forso-called hybrid vehicles that is used in combination with a combustionengine such as a gasoline engine and a diesel engine.

REFERENCE SIGNS LIST

-   1: Coin type battery-   2: Case-   2 a: Positive electrode can-   2 b: Negative electrode can-   2 c: Gasket-   3: Electrode-   3 a: Positive electrode-   3 b: Negative electrode-   3 c: Separator

What is claimed is:
 1. A method of producing a positive electrode activematerial for nonaqueous electrolyte secondary batteries, comprising awater washing step of mixing, with water, a lithium-nickel compositeoxide powder represented by a general formula:Li_(z)Ni_(1−x−y)CO_(x)M_(y)O₂ (where 0≤x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30are satisfied, and M is at least one element selected from Mn, V, Mg,Mo, Nb, Ti, and Al) and having a layered crystal structure composed ofprimary particles and secondary particles formed by aggregation of theprimary particles to form a slurry, and washing the lithium-nickelcomposite oxide powder with the water, and then subjecting the slurry tosolid-liquid separation to obtain a washed cake constituted by washedlithium-nickel composite oxide particles; a mixing step of mixing atungsten compound powder not containing lithium with the washed cake toobtain a tungsten-containing mixture; and a heat treatment step ofheating the tungsten-containing mixture, wherein the heat treatment stepincludes: a first heat treatment step of heating the tungsten-containingmixture to allow a lithium compound present on a surface of the primaryparticles of the washed lithium-nickel composite oxide to react with thetungsten compound so as to dissolve the tungsten compound therein,thereby forming lithium-nickel composite oxide particles with tungstendispersed on the surface of the primary particles; and subsequent to thefirst heat treatment step, a second heat treatment step of performingheat treatment at a higher temperature than in the first heat treatmentstep to form lithium-nickel composite oxide particles with a lithiumtungstate compound formed on the surface of the primary particles of thelithium-nickel composite oxide.
 2. The method of producing a positiveelectrode active material for nonaqueous electrolyte secondary batteriesaccording to claim 1, wherein the slurry in the water washing step has aconcentration of 500 to 2500 g/L.
 3. The method of producing a positiveelectrode active material for nonaqueous electrolyte secondary batteriesaccording to claim 1, wherein the slurry in the water washing step has atemperature of 20 to 30° C.
 4. The method of producing a positiveelectrode active material for nonaqueous electrolyte secondary batteriesaccording to claim 1, wherein the washed cake obtained in the waterwashing step has a water content controlled to 3.0 to 15.0 mass %. 5.The method of producing a positive electrode active material fornonaqueous electrolyte secondary batteries according to claim 1, whereinthe tungsten compound not containing lithium used in the mixing step istungsten oxide (WO₃) or tungstic acid (WO₃.H₂O).
 6. The method ofproducing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein an amountof tungsten contained in the tungsten-containing mixture is 0.05 to 2.0at % with respect to the total number of atoms of Ni, Co, and Mcontained in the lithium-nickel composite oxide particles.
 7. The methodof producing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein the heattreatment step is performed in any one atmosphere of decarbonated air,inert gas, and vacuum.
 8. The method of producing a positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto claim 1, wherein the first heat treatment step is performed at a heattreatment temperature of 60 to 80° C.
 9. The method of producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to claim 1, wherein the second heat treatment stepis performed at a heat treatment temperature of 100 to 200° C.